In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicants expressly reserve the right to demonstrate that such structures and/or methods do not qualify as prior art.
The use of oxide ceramics as a framework material for dental restorations has long been state of the art. This material is characterized by an excellent biocompatibility and outstanding mechanical properties. For many years it has also been widely used as an implant material and for prostheses. In the past few years, ceramics based on partially stabilized ZrO2 ceramics have been used in particular.
The shaping of these ceramics in dental engineering is typically performed by mechanical means. In particular, milling of partially sintered ceramics with CAD/CAM processing units has gained acceptance. The shrinkage which occurs during final densification of shaped bodies, going from a density of approximately 40-60% to a density of more than 95% is taken into account during the mechanical processing. The quoted density is relative to the respective theoretical density.
The disadvantage of the ZrO2 ceramics is the low translucency and their milky-white colour. A non-coloured and non-coated restoration or restoration part looks like an unnatural tooth. Colouring the ZrO2 ceramic to match the patient's situation for an aesthetic tooth reconstruction is thus essential.
A particularly great disadvantage of sintered ceramics according to the state of the art is that they do not produce blanks for CAD/CAM processing in open-pored form or in dense-sintered form which are multi-coloured or have zones of different colours corresponding to the coloration of a natural tooth.
All that is known from the state of the art is a series of technical solutions for coloured, not multi-coloured, blanks. However, these solutions have the disadvantage that the natural tooth colour, the colour gradient, the polychromatism, the graduated translucency and brightness of colour were not achieved. These known solutions are described as follows:
The preparation of an open-porous coloured and white Y2O3-containing ZrO2 blank is achieved according to EP 1 210 054 from liquids via co-precipitation from chlorides which contain Zr, Y, Al, Ga, Ge, In, Fe, Er and Mn ions. By means of the co-precipitation and subsequent calcination, the prepared powder already contains the colouring ions before shaping. Oxides from the group Fe2O3, Er2O3 and MnO2 are selected as colouring compounds. The disadvantage of this invention is that a very costly and laborious method of the co-precipitation process with subsequent calcination must be carried out in order to obtain a coloured powder. This means that this must be carried out for every single colour.
In the following disclosures monolithic ceramics are presented which each allow one specific colour, thus no polychromatism, to be achieved:
U.S. Pat. No. 5,263,858 (Yoshida et el.) describes the preparation of ivory-coloured shaped bodies for dental applications (brackets), wherein during the preparation of the stabilized ZrO2 ceramic colouring compounds in solutions are added before the calcination or powdery mixtures of colouring oxides after the calcination. In order to achieve the desired ivory shade, the addition of Fe2O3, Pr6O11 and Er2O3 is necessary. However, this process has the disadvantage that it is a multi-stage process.
It is further known from the state of the art according to FR 2 781 366 to mix the colouring components with the starting powder of the ZrO2, grind and sinter jointly. Fe2O3, CeO2 and Bi2O3 are mentioned as colouring oxides.
EP 0 955 267 mentions contents of 5-49 wt.-% CeO2, whereby a colouring is achieved.
For the preparation of completely cubically stabilized zirconium dioxide in the arc-furnace process, according to EP 1 076 036 B1 one or more stabilizing and colouring oxides or their precursors are added to a ZrO2 source. The colouring oxides of the elements Pr, Ce, Sm, Cd, Tb are inserted into the crystal lattice of the ZrO2 after the sintering process.
U.S. Pat. No. 5,656,564 relates to the preparation of zirconium oxide shaped bodies which contain oxides of the rare earths boron oxide, aluminium oxide and/or silicon oxide. The shaped bodies contain the zirconium dioxide as a mixed phase of tetragonal and monoclinal ZrO2. Oxides of the elements Pr, Er and Yb are introduced into the sintered ceramic as colouring oxides.
Technical solutions are further known according to the state of the art which allow coloured blanks to be obtained by infiltration of liquids. However, these technical solutions have the serious drawback that a colouring takes place after the pre-sintering process and thus liquids are introduced into an open-porous ceramic body. The colouring is thus not completely homogeneous and also a multi-coloration cannot be achieved.
Unlike sintered ceramics, such as ZrO2 and Al2O3, a process for the preparation of multi-coloured glass ceramic blanks is known in the materials group of the glass ceramics (DE 197 14 178 C2). However, the preparation of multi-coloured ZrO2 blanks is not mentioned in this invention.
A disadvantage of the known solutions from the state of the art is that multi-coloured sintered ceramic blanks cannot be prepared. Moreover, the solutions according to the state of the art are very costly and quality problems arise. The latter applies e.g. to the infiltration technique, in which due to the subsequent colouring of a partially sintered blank or of a shaped dental product only the voids (pores) between the partially sintered particles of the starting powder can be occupied by the colouring ions. As a result, also only discrete areas of the surface of the particles are coloured with a layer of the colouring oxides, a continuous coverage of the surface of the particles of the starting powder not being possible. A further great disadvantage with an infiltration is the concentration gradient of the colouring from the outside inwards. If a porous body is introduced into the colouring solution, the starting solution releases part of the dissolved colouring ions, starting from the outside inwards, and thus the colouring solution is “depleted” of some of its colouring substances. The consequence of this is that there is a higher concentration of the colouring ions or then oxides outside than in the inside of the shaped body. Furthermore, only a certain depth of penetration can be achieved by means of the infiltration technique.